Google is your friend here. Biggerpockets.com has a ton of good information, also check the RE forum here at MMM.
Basic rules are usually:
-Rent should be 1-2% of total purchase price per month.
-Costs will be ~50% of gross rent (so if you're financing the deal, half of your rent minus P&I is your monthly cash flow, roughly)
This is all assuming you are going to buy/hold and want immediate cash flow. If you are flipping, or have an inside line that you'll see a ton of appreciation, or are doing some mix of strategies (ie live in one side of a duplex and rent the other half or something along those lines) the "rules" can be bent or broken in some cases. Just don't fall into the (I fell in once, long story...) "Rent minus mortgage and taxes = profit!" trap.
-W